Approximately 50 to 85% of patients with end-stage renal disease on chronic hemodialysis (HD) have sleep complaints and/or sleep disorders-problems that are associated with decreased quality of life and functional health status. Thus, population specific interventions are needed that address both the underlying mechanisms and consequences of these abnormalities. We propose to test a novel application of an existing intervention-cool dialysate (often used to treat hypotension) as opposed to warm dialysate (standard treatment) during HD-for its ability to stabilize the sleep/wake cycle of chronic HD patients. Primary outcome variables include objective measures of nocturnal sleep and daytime sleepiness and selected sleep-related physiological, psychological, behavioral, and general health outcomes. A randomized, single-blinded (investigators) control group, experimental design will be used. Sixty-eight chronic HD patients, age 21 to 70 years, will be recruited and randomized into one of two groups, Group A (experimental) and Group B (control). For individual subjects, the study will (be) conducted over a 9-month period of time and divided into 3 phases-Phase I (3 months), Phase II (3 months), and Phase III (3 months). During Phase I, both groups of subjects will receive standard treatment (warm dialysate) and data regarding nocturnal sleep (objective and subjective measures of sleep latency, sleep efficiency, total sleep time, and sleep disturbance), daytime sleepiness (physiologic, manifest, and introspective), body temperature (distal/proximal skin gradient and axillary body temperature rhythms), psychological (mood), behavioral (daily activity/rest), and general health outcomes (quality of life and functional status) will be collected. During the next 3 months (Phase II), Group A will receive HD with cool dialysate while Group B will continue to receive standard treatment. During Phase III, both study groups will return to baseline treatment and receive HD with warm dialysate. In each Phase, data on all outcomes of interest will be collected. Data analysis will provide important information regarding the impact of this intervention on the sleep/wake cycle of HD patients and other related clinical outcomes. In addition, study results will make a significant contribution to the scientific understanding of how medical treatments may interact with normal systems to induce sleep/wake abnormalities.